Fluorescent dye additives have been utilized as tracers to detect leaks in fluid systems. The tracer is combined with a system operating fluid such as lubricants, hydraulic fluids, heat transfer fluids, and refrigerants. The dye additive fluoresces when illuminated by suitable light having wavelengths in the rage of 300 to 450 nanometers.
Naphthalimide diester tracer dyes for leak detection are described in U.S. Pat. No. 6,248,890. The dyes are miscible in a variety of polar and non-polar organic liquids such as refrigerant lubricants and refrigerants, mineral oil, polyalkylene glycols, coolants, polyolesters, motor oil, gear oil, transmission fluid, synthetic oils, and polyalphaolefins. They are used to detect fluid system leaks. The naphthalimide diester dyes are highly fluorescent. They possess excellent solution stability during storage and operation, and is stable at high temperatures.
The naphthalimide diester tracer dye compositions described in U.S. Pat. No. 6,248,890 are prepared by esterification of naphthalimide diols with fatty acids. The reaction product mixture inevitably contains substantial levels of unreacted fatty acid. Fatty acid contamination of naphthalimide diester tracer dye leads to corrosion of susceptible fluid system components. Acid-induced chemical degradation of system components can lead to particulate build-up and ultimately restricted flow and clogged systems. High levels of corrosion may lead to structurally weakened parts and equipment, and high failure rates. Unreacted fatty acid must therefore be removed from naphthalimide diol esterification product by further reaction with acid scavenging agents, to reduce the product composition acid number to acceptable levels. Typically, the acid scavenger is an epoxy compound, e.g., a monofunctional or difunctional glycidyl ether or mixtures thereof. The epoxy compound is typically added to the esterification reaction product mixture following reaction of the reactant diol and fatty acid under conditions of elevated temperature which causes excess fatty acid to react with epoxy to form an alcohol. Such reaction conditions include, for example, reaction times from 2-24 hours at temperatures of from 100° C. to 160° C. to react. See U.S. Pat. No. 6,248,890. This further reaction step adds significantly to the cost of fluorescent naphthalimide diester dyes that are to be used as leak detection tracers. Most importantly, significant levels of unreacted fatty acid remain even after treatment with epoxy compounds.
What is needed is a process for producing fluorescent naphthalimide diester dye from naphthalimide diols that avoids contamination by unreacted fatty acid and the need for subsequent scavenger treatment to convert unreacted acid. What is needed is a process for preparing naphthalimide diester dye compositions from naphthalimide diol that avoids or minimizes the use of fatty acid reactants, thereby resulting in a low acid value naphthalimide diester dye composition without subsequent acid scavenging. What is needed is an improved fluorescent naphthalimide diester dye composition that is characterized by not only low crystallinity, but also low acid content, and is also free of acid scavenger reaction products, such as alcohols resulting from the scavenger reaction between fatty acids and epoxy compounds, which provide no useful purpose in naphthalimide diester dye composition.